VSE: How to set a custom Proxy percentage size; for example 10%
No, you can only choose between 25, 50, 75 and 100%.

It's hard coded as you can see when you try to autocomplete the following in a python console:

&gt;&gt;&gt; bpy.ops.sequencer.enable_proxies(
enable_proxies()
bpy.ops.sequencer.enable_proxies(proxy_25=False, proxy_50=False, proxy_75=False, proxy_100=False, override=False)